lec 19.doc

Chapter 9- Quantitative Genetics
• Quantitative Genetics- the branch of evolutionary biology that provides tools for analyzing the evolution
of multi-locus traits
9.1- The Nature of Quantitative Traits
• Qualitative Traits- characteristics that we can assign to individuals by just looking at them, or perhaps by
conducting a simple genetic test
• Traits with discrete phenotypes are special examples; most traits in most organisms show continuous
variation
o Such as height, athletic ability, and intelligence
o Also beak length in soapberry bugs and bill depth in medium ground finches
• Traits with continuous variation cannot assign individuals to discrete phenotypic categories by simple
inspection
o Measurements must be taken
o Characters with continuously distributed phenotypes are called quantitative traits
 Are determined by the combined influence of:
• Genotype at many different loci
• & The environment
• Quantitative traits are traits for which the distribution of phenotypes is continuous rather than discrete
• Quantitative traits are consistent with Mendelian genetics. They are influenced by the combined effects of
the genotype at many loci. Quantitative traits are also influenced by the environment.
9.3- Measuring Heritable Variation
• Basic tenets of Darwin’s theory of evolution by natural selection:
o If there is heritable variation among the individuals in a population, and if there are differences in
survival and/or reproductive success among the variants, then the population will evolve.
• Quantitative genetics includes tools to measure heritable variation, tools for measuring differences in
survival and/or reproductive success, and tools for predicting the evolutionary response to selection
• Quantitative genetics allows us to analyze evolution by natural selection in traits controlled by many loci
• The first step in a quantitative genetic analysis is to determine the extent to which the trait in question is
heritable. That is, we must partition the total phenotypic variation (V p into a component due to genetic
variation (VG) and a component due to environmental variation (V ) E
• Heritability- fraction of the total variation in a trait that is due to variation in genes
o In the broad sense, that fraction of the total phenotypic variation in a population that is caused by
genetic differences among individuals; in the narrow sense, that fraction of the total variation that is
due to the additive effects of genes.
• Phenotypic Variation (V )- tPtal variation in a trait
• Genetic Variation (V )- Gariation among individuals that is due to variation in their genes
• Environmental Variation (V )- vaEiation among individuals due to variation in their environments
• Broad-Sense Heritability or Degree of Genetic Determination:
Heritability = V G VP= V /G(V G V ) E
Estimating Heritability from Parents and Offspring
• If the variation among individuals is due to variation in their genes, then offspring will resemble their
parents
• Figure 9.13:
o [9.13a] If offspring do not resemble their parents, then the slope of the best-fit line through the data
will be near 0; this is evidence that the variation among individuals in the population is due to
variation in their environments, not variation in their genes
o [9.13c] If offspring strongly resemble their parents, the slope of the best-fit line will be near 1; this is
evidence that variation among individuals in the population is due to variation in their genes, not
variation in their environments
o [9.13b] Most traits in most populations fall somewhere in the middle, with offspring showing a
moderate resemblance to their parents; this is evidence that the variation among individuals is
partly due to variation in their environments and partly due to variation in their genes
• Narrow-sense Heritability (h )- that fraction of the total phenotypic variation in a population that is due
to the additive effects of genes
o Is an estimate of the fraction of the variation among the parents that is due to variation in their
genes
2
h = V AV P V /A(V +AV + D ) E
• Heritability, h , is a measure of the (additive) genetic variation in a trait
• Additive genetic variation (V ) is variation among individuals due to the additive effects of genes
A
• Dominance genetic variation (V ) is vDriation among individuals due to gene interactions such as
dominance o total genetic variation is the sum of the additive and dominance genetic variation
 V G V +AV D
• Narrow sense heritability, h , allows us to predict how a population will respond to selection
Estimating Heritability from Twins
• Mono